The rates of biologically relevant reactions of iron porphyrins and hydroporphyrins will be determined by visible and NMR spectroscopy and by chronoamperometry. The reactions that will be studies are outer-sphere electron transfer, inner-sphere electron transfer, and ligand exchange. Structural analysis by single crystal X-ray diffraction techniques will add to the interpretation of the chemical significance of the kinetic differences. The tetrapyrrole macrocycles that will be studied are derived from octoethylporphyrin and tetraphenylporphyrin. These efforts are aimed at understanding why some reductase enzymes contain iron chlorins or an iron isobacteriochlorin (siroheme) as substrate binding and activating sites instead of iron porphyrins. The enzymes in question, sulfite-and nitrite-reductases, catalyze the efficacious yet poorly understood multi-electron reductions of sulfite to sulfide, nitrite to ammonia, and oxygen to water. The organisms that rely on these enzymes for the assimilation of sulfur and nitrogen (plants and bacteria) or for anaerobic respiration (specialized bacteria0 make these elements available in their reduced, more readily utilized forms to the rest of the biosphere. All animals depend directly or indirectly on this source of these essential metabolites. This research is an example of the model approach to bioinorganic chemistry. The long range goal is to discover what special features of the chemistry of iron hydroporphyrins make them better suited than iron porphyrins at mediating these multi-electron reductions.